Electrostatic capacitance is a method of energy storage that has not been widely used for bulk electrical energy storage. As a method of energy storage, electrostatic capacitors have excelled at the speed with which they can accumulate and discharge energy. The physical mechanisms by which such energy storage can take place are widely documented and described. In general the charge and discharge mechanisms for traditional electrostatic energy storage in a dielectric material is in a time-domain regime of picoseconds to hundreds of microseconds.
There has been a recent trend in the use of electrochemical capacitors for enhanced storage of electrical energy. These capacitors derive their enhanced characteristics from two primary mechanisms: double layer capacitance and pseudocapacitance. Double layer-type capacitors use an electrical double layer (explained below) to achieve a very small charge separation (d), which increases electric field (E) for a given voltage, increases capacitance (C) and consequently increases the energy stored (U) for the given voltage versus a conventional planar surface capacitor, as apparent in Eqs. 1 through 3 below.
                    E        =                  V          d                                    Eq        .                                  ⁢        1            where E=electric field, V=potential difference or voltage, and d=separation of charged plates.
                    C        =                              k            ⁢                                                  ⁢                          ɛ              0                        ⁢            A                    d                                    Eq        .                                  ⁢        2            where k=relative permittivity or dielectric, C=capacitance, ∈0=permittivity of free space, and A=cross-sectional surface area.
                    U        =                              1            2                    ⁢                      CV            2                                              Eq        .                                  ⁢        3            where U=energy stored, C=capacitance and V=voltage.
Practically, the smaller thickness (d) allows for much more surface area of the plates to be packaged (usually rolled or stacked) in a given volume. As evident from Eq. 2, this area increase also significantly increases capacitance. Devices of the above described nature are commonly referred to as electric double layer capacitors (EDLCs).
In pseudocapacitors, which are a hybrid between double-layer capacitors and batteries, both the bulk and the surface of the material play key roles. They thus can store much more energy than conventional planar surface capacitors, but face many of the same reliability and scientific challenges as advanced batteries, including high cost due to expensive raw materials and complex processing. Pseudocapacitance imitates battery technology by storing energy in chemical reactions (oxidation and reduction) which take place at or very near the surface of the relevant electrodes. The surface nature of the reactions is the distinguishing characteristic from chemical battery technology. Either or both of these effects (i.e., double layer and pseudocapacitance) may be used in so called “supercapacitors.”
Current EDLCs can handle only low voltages before breakdown. In order to attain the higher voltages necessary for many practical applications (such as electric vehicles), low voltage EDLCs are connected in series much in the same way batteries are series-connected for high voltage use.
A need exists for energy storage devices with greater storage capacity, that can be connected in series, and that are capable of handling higher voltages. A need also exists for a method for discharging such energy storage devices.